1. Field of the Invention
This invention relates to pneumatic tires for heavy load vehicles and more particularly to a highly durable pneumatic tire comprising a bias laid carcass body composed of a plurality of stacks each including at least two superimposed rubberized organic fiber cord ply groups which are different in cord diameter and mainly used for trucks, trailers, construction vehicles, agricultural vehicles, industrial vehicles, airplanes or the like.
2. Description of the Prior Art
The above mentioned kind of tire comprising a bias laid carcass body composed of a plurality of stacks each including at least two superimposed rubberized organic fiber cord ply groups which are different in cord diameter has a number of advantages, but has a few disadvantages. The most important disadvantage is that the number of carcass plies must be substantially increased for the purpose of obtaining strength required for the condition under which the tire is used. This fact will now be described with reference to an example.
In a nylon cord of 1,260 denier/2 strands which has most widely been spread in the tire field, the number of carcass plies is usally increased up to 10 or 20 and exceeds 40 when the tire size becomes large. It is clear that the bias tire having such large number of superimposed carcass plies is not desirous in view of production efficiency and manufacturing cost if compared with a steel radial tire which could use only one ply even when the tire size is considerably large.
In order to decrease the number of plies with the required strength of the bias tire maintained, an easy-going method of increasing a cord diameter and the number of filaments correspondingly to increase the strength per 1 cord has heretofore been proposed. For example, the use of 1,890 denier/2 strands instead of 1,260 denier/2 strands ensures an increase of the cord strength up to 1.5 times stronger than that of the latter, thereby decreasing the number of plies to about 0.75 times smaller than that of the latter. It is a matter of course that the cord strength of the 1,890 denier/2 strands is not increased up to 1.5 times stronger than that of 1,260 denier/2 strands because the number of cords must be decreased in order to maintain a necessary gap between the cords. The conventional method of decreasing the number of carcass plies is capable of decreasing 30 plies to the order of 20 plies.
However, in the conventional method of decreasing the number of carcass plies, there is a risk of the tire being subjected to a cord breaking up failure which is a vital tire failure.
The cord breaking up failure shall be understood to mean breakage of cords of the carcass ply due to certain causes. The cord breaking up failure seldom occurs when the tire is used under appropriate conditions. But, an extreme decrease of an internal pressure of the tire or heavy load or sudden shock subjected to the tire results in the cord breaking up failure. In addition, if the strength of the carcass becomes small when the tire size is erroneously selected or the like, the cord breaking up failure becomes induced.
Investigations and experimental tests have demonstrated the result that, with the carcass strength be the same, the cord breaking up failure which has not so often been induced when a small cord diameter is used becomes so often induced when a large cord diameter is used.